A wide variety of membrane filtration systems have been used for many years to treat contaminated water, such as, for example, sewage or waste water. Such systems vary in complexity and cost. In an effort to make the treatment processes more cost efficient, submerged membrane filtration processes have been developed in which membrane modules including hollow fiber filtration membranes are submerged in a large tank, and filtrate is collected by way of suction applied to the filtrate side of the hollow fiber membranes. This results in suspended matter collecting on an external side of the hollow-fiber membrane surface, which reduces filtration performance. Thus, an effective method of removing the matter from the surfaces is required.
The effectiveness and viability of these membrane systems largely depend on having effective ways to clean the surfaces of the hollow fiber membranes, so that they do not become clogged and/or lose their effectiveness. Common methods of cleaning include backwash using a liquid permeate and/or gas, relaxing, chemical cleaning, and membrane surface aeration using a gas in the form of bubbles. In a gas aeration system, a gas is introduced into the base of the membrane module. The bubbles then travel upwards to scour the membrane surface to remove the fouling substances formed on the membrane surface. The shear force produced largely relies on the initial gas bubble velocity, bubble size, and resultant forces applied to the bubbles. To enhance scrubbing, more gas has to be applied. However, energy consumption increases as the volume of gas increases. For applications where the liquid being treated has large quantities of suspended matter, the gas aeration system is susceptible to becoming blocked.
One way to reduce energy consumption, while still obtaining efficient membrane cleaning, is cyclic aeration (e.g., small bubble dispersed aeration). Cyclic aeration systems provide gas bubbles on a cyclic basis, instead of a continuous basis. In order to provide for such cyclic operation, such systems normally require complex valve arrangements and control schemes, the cost of which offsets the operational savings of a cyclic system. In addition, cyclic aeration systems can have a limited range of air flow rate operation limiting the ability to reduce operational cost. For example, issues may arise with cyclic aeration systems when the air flow rate is turned down below a minimum threshold. Such issues may include, for example, insufficient circulation of the liquid within the membrane tank, insufficient scouring of the membrane surface leading to fouling and sludging, and increased probably of clogging of aeration system components (e.g., nozzles and distribution piping).
Another option to reduce energy consumption, is to have a pulsed air-lift system similar to that described in U.S. Pat. No. 8,287,743 (the '743 patent) to Zha et al. According to the '743 patent, the system includes membrane modules that have a pulsed gas-lift pump device provided below a distribution chamber of the membrane modules. The pulsed gas-lift pump device is configured to receive gas from a pressurized source, which displaces feed liquid within a gas collection chamber of the pulsed gas-lift device until it reaches a certain level. Once the volume of gas reaches a certain level the gas breaks the liquid seal and is discharged in the form of bubbles through the distribution chamber and into the base of the membrane module. The discharge of gas also sucks feed liquid through the pulsed gas-lift pump producing a two-phase gas/liquid pulse designed to scour the surfaces of the membranes.
The system and method of the '743 patent may provide some benefits in some applications. However, it may have certain drawbacks and inefficiencies, for example, the bubble formed by the pulsed air-lift can deform or shift as it moves up the membrane module, thereby reducing scrubbing efficiency. The disclosed embodiments may help solve these drawbacks and inefficiencies as well as other problems.